Passive microwave remote sensing of the earth's surface provides an opportunity for monitoring the presence of water in many forms. These measurements can be made regardless of cloud cover and in total darkness. Some of the parameters that can be measured on land are soil moisture and soil temperature, the amount of vegetation, snow cover extent and snow water equivalent. In the ocean areas, the sea surface temperature, wind speed, sea ice type and concentration, and hurricane positions can be measured.
Many of these applications require that the instruments have a spatial resolution of 1 to 5 kilometers and operate at frequencies down to 1.4 GHz. For example, in order to measure soil moisture, the frequency has to be low enough for the microwaves to penetrate through the vegetation cover and into the soil.
Most existing imaging microwave radiometers have conventional parabolic dish antennas that scan conically by rotating about the nadir axis. Such conical scanning produces a constant incidence angle at the earth's surface; this is necessary for many remote-sensing tasks. However, it would be impractical to build a scanning dish antenna of this type that is large enough to get 5 kilometer spatial resolution at 1.4 GHz. This would require spinning a 12-meter antenna faster than one revolution per second. It is easy to appreciate the many problems inherent in placing such a structure in orbit. Such a structure would require a system for compensating for the angular momentum of the spinning antenna. This requires a counter-spinning structure that: adds mass to be launched into orbit; consumes energy due to bearing loses; and poses the potential for loss of the satellite if the bearings seize. This type of structure would have to have sufficient structural integrity to withstand the forces developed by such a large, rapidly rotating structure.
Consequently a need exists for a microwave radiometer that can achieve acceptable radiometric sensitivity and spatial resolution without requiring any moving parts.